The known process of plasma or gaseous ionic carburizing is used to produce a hard surface layer on steel parts. A known process of plasma carburizing involves heating a workpiece in a vacuum furnace to a temperature in the range of about 850-1000.degree. C. The vacuum furnace outer wall is connected to a ground potential whereas the workpieces in the furnace chamber are connected to a source of large negative voltage. When the negative voltage source is energized an electric field is established in the furnace chamber. The workload of steel parts is effectively a cathode and the furnace chamber wall is the anode. A process gas formed of a gaseous mixture containing a carbonaceous gas such as propane or methane flows through the furnace chamber at subatmospheric pressure. The carbonaceous gas is ionized by the influence of the strong electric field. Positive carbon ions are drawn to, absorbed, and diffused into the surface layer of the cathodic steel workpiece.
The workpieces must be insulated from the anodic furnace chamber wall to prevent short circuiting of the electrical system. The workload is usually loaded onto a workpiece table which is supported on one or more electrodes. During the plasma carburizing process carbon soot is deposited on the surface of the ceramic insulators used in the electrodes. The carbon soot develops from both thermal and ionic breakdown of the carbonaceous gas. In time, the carbon soot deposited on an electrode builds up to a level which causes a short circuit over the ceramic insulator, resulting in interruption of the process. Significant and undesirable time delays can occur while the soot is cleaned from the insulators.